fw/services/common/light: fix dynamic backlight integer underflow

When user max intensity is set below 10% (the hardcoded low_intensity
threshold), the dynamic backlight calculation would cause an integer
underflow. The subtraction (user_max_intensity - low_intensity) wraps
around in unsigned arithmetic, producing a very large value that results
in brightness exceeding BACKLIGHT_BRIGHTNESS_MAX (appearing as >100%).

Fix by clamping user_max_intensity to low_intensity minimum before
performing the calculation.

Fixes backlight showing 187% when max intensity set to LOW (5%) on
Obelix PVT 4.9.110.

Signed-off-by: Joshua Jun <joshuajun@proton.me>

Author: jplexer

src/fw/services/common/light.c

@@ -115,6 +115,12 @@ static uint16_t prv_backlight_get_intensity(void) {
     const uint32_t min_light_threshold = backlight_get_dynamic_min_threshold();
     const uint32_t max_light_threshold = backlight_get_dynamic_max_threshold();
 
+    // If user max intensity is below low intensity, clamp to low intensity
+    // to prevent underflow in the calculation below
+    if (user_max_intensity < low_intensity) {
+      user_max_intensity = low_intensity;
+    }
+
     // If below minimum threshold, return low intensity
     if (light_level < min_light_threshold) {
       return low_intensity;